The present invention relates to DC restoration circuits used in television receivers and, more particularly, to a DC restoration circuit employing current-mode feedback techniques to restore the DC level of a received television signal.
The video component of a transmitted television signal is characterized by a DC level establishing or defining the average background brightness of a scanned scene. In order for the reproduced picture at the receiver to have the proper background brightness, i.e. the proper level with respect to black, the transmitted DC component must be retained or otherwise restored in the receiver. In television receivers employing direct coupling techniques throughout the entire video section, the DC component of the composite video signal is inherently retained in the receiver and no special circuits are necessary to insure the reproduction of a picture having a proper average background brightness level. However, for purposes of simplicity and economy, as well as to prevent the accumulation of system errors, most current television receivers have video sections incorporating RC coupling circuits. As a result of the DC blocking action of the coupling capacitors in these circuits, the background brightness determining component of the video signal undergoes rather radical level shifts and must therefore be suitably restored in the receiver to enable reproduction of an acceptable picture.
Typically, the DC component of the video signal in an AC coupled video section is restored to establish a suitable average background brightness level in response to a manually operable brightness control which is used in association with a clamping capacitor to set the picture tube grid bias voltage. More specifically, the brightness control is used to establish an adjustable reference potential, a predetermined portion of the horizontal blanking interval of the video signal, e.g. the back porch interval, being clamped to the reference potential by the clamp capacitor to establish a desired black level. Thus, in response to adjusting the brightness control, the viewer effectively controls the black level, and thereby the average background brightness, of the reproduced picture by restoring the DC component of the video signal to a proper level. In this regard, while the back porch, as well as the front porch, of the horizontal blanking interval does not precisely correspond to black level as defined by FCC standards, the difference therebetween is considered to have only a negligible effect on DC restoration circuits of the foregoing type.
An embodiment of a DC restoration circuit employing the above principles is illustrated in an article entitled "A Versatile Low-Level Luminance IC for TV" appearing on pages 169-175 of the August, 1978 issue of IEEE Transactions on Consumer Electronics, Vol. CE-24, No. 3. In the illustrated circuit, the unblanked luminance component of the video signal is coupled to one input of a comparator while the second input to the comparator consists of a DC reference voltage derived from a manually adjustable brightness control. A back porch gate pulse enables the comparator for directing current flow into or out of a clamp capacitor which sets the DC voltage levels in a feedback loop for adjusting the black level of the luminance signal until a condition of equality between the DC reference voltage and the level of the back porch is established. While this circuit is adequate in certain applications, its operation is characterized by a rather limited dynamic range rendering the circuit unsuitable for use with different types of television receivers. The limited dynamic range characterizing the circuit is primarily attributable to the fact that the feedback loop is used to couple voltage signals for controlling system operation.
It is therefore a primary object of the present invention to provide a new and improved DC restoration circuit capable of widespread application and exhibiting a rather large dynamic range. As explained in detail hereinafter, these objects are achieved through the provision of a novel bi-directional gated back porch clamp circuit utilizing current-mode feedback techniques for controlling the black level of the luminance component of a composite video signal.